The Salton Sea was created when irrigation routes were mishandled and allowed to run into the Salton Sink in southeastern California from 1905 to 1907. The inland sea has no outlet and is fed by agricultural runoff. The resulting high salinity is responsible for the deaths of fish and birds. The sea also has a high level of selenium.
Between the 1920s and 1950s, the Salton Sea served as a vacation area of sorts, but the agricultural runoff created a nutrient-rich medium in which algal blooms formed and killed off millions of fish. Birds have also fallen victim to this stagnant body of what can only dishonestly be described as water.
The biggest worry about the sea, however, is that the hazardous chemicals that are currently underwater will be exposed to the air. The resulting dust storms could greatly affect the air quality in Los Angeles. Owens Lake (ranked no. 9) is an example of what could happen, but estimates based on the shallowness of the sea and its surface area could make this a dust-bowl area to rival that of the 1930s, as depicted in The Grapes of Wrath.
The sheer magnitude of this threat, combined with the cost of reclamation efforts, makes the Salton Sea a serious environmental disaster that could get worse if it is not dealt with properly, especially when you consider its proximity to the second-largest city in the United States.